1. Field of the Invention
This invention relates to the field of gas purification, and more specifically, to a system and process for on-site generation, purification, and distribution of ultra-pure ammonia.
2. Description of the Prior Art
As semiconductor integrated devices become smaller and the devices using semiconductor integrated devices become more sophisticated, there are increasing demands for the physical and chemical properties of the actual semiconductor material to have properties nearer to the ideal and intrinsic properties of the semiconductor material. The manufacture of semiconductors involves the use of reactive gases that are composed of various elements. In addition, manufacturing processes such as metal-organic chemical vapor deposition (MOCVD) and other related manufacturing techniques are used in the manufacture of semiconductors. In these processes, the purity of the reactive gases plays a large part in determining the resulting quality of the semiconductor device being manufactured, and in particular, the electronic quality and characteristics of the manufactured semiconductor device. Consequently, there is an increasing demand in the microelectronics industry for ultra-pure process gases, and to meet these demands, methods for ultra-purification of gases useful in microelectronics processes have experienced extensive technological effort and advances.
Ultra-pure ammonia is an example of the gases in demand by the semiconductor industry as the use of ultra-pure ammonia in semiconductor and compound semiconductor device manufacturing has gained ascendancy in the past decade. Specifically, ultra-pure ammonia is used in nitride manufacturing processes for the production of high brightness blue and white LEDs (light emitting diodes), high performance optoelectronics, and other electronic devices. As device geometries continue to shrink and LED brightness demands increase, the need for continued contaminant level reduction as well and consistent delivery quality is likely to remain critical to manufacturing capacities and yields. Adding complexity to ammonia delivery installations is the recent need to sustain high flow rates. To aid this trend, ammonia delivery systems have moved from cylinder quantities, e.g., with a nominal capacity of about 23 kg and ton container quantities, e.g., with a nominal capacity of 220 kg, to ISO (International Organization of Standardization) module quantities having much larger capacities, e.g., up to 15,000 kg or larger capacity.
Certain large manufacturers are embarking on greater use quantities that will quickly reach the practical limitations of existing ultra-pure ISO module deliveries. Maintaining purity in high flow systems from the large containers to the use points is problematic in that frequent connections, module change-outs, and purge sequences are required. Multiple ISO modules are necessary to meet flow demands and as such large abatement systems are required to mitigate release scenarios. Additionally, regulating authorities are imposing strict permitting processes, including complete building containment and abatement of ISO module quantities. An alternative large volume supply scenario includes stationary storage tanks from which raw ammonia is delivered to the site, purified, and distributed to the use points. This eliminates the large rolling stock of high purity ISO modules and also reduces the contamination potential at trailer connection and disconnection. However, the abatement and permitting requirements are still gating or limiting factors in this type of installation that prevent these installations from meeting the growing demands for ultra-pure ammonia.
There are several chemical processes that are used to manufacture ammonia. The three most prevalent methods include the Haber-Bosch process, indirect electrochemical dissociation, and urea decomposition. The Haber-Bosch process reacts gaseous hydrogen and nitrogen over a metal catalyst at high temperatures (e.g., at 475° C.) and pressures (e.g., at 20 MPa). This process is a proven large-scale industrial process; however, it uses harsh conditions and has not been proven technically or economically effective below the ton/hour range. The electrochemical dissociation process has been proposed by some in the semiconductor industry as an alternative to the Haber-Bosch process for the generation of ammonia. This process also reacts hydrogen and nitrogen. However, it is an indirect synthesis via a molten alkali-metal halide electrolyte with nitrogen introduced at the cathode and hydrogen introduced at the anode. The electrochemical dissociation process also operates at elevated temperatures (e.g., at 400° C.) but at ambient pressure. While utilizing less harsh operating conditions or parameters than the Haber-Bosch process, the electrochemical dissociation process has not been proven above pilot scale production rates and has a high risk of alkali metal contamination. Another concern with adopting these two processes for generating ultra-pure ammonia it that the Haber-Bosch and electrochemical distribution processes require large amounts of hydrogen, which adds significantly to the risk of operating an ammonia generation facility. Another process for generating ammonia is through the dissociation of urea. Although this reaction has been known in the art, it has recently been incorporated into a complete packaged plant designed for abatement systems for NOx reduction but has not been tested or used for the production of ultra-pure ammonia in the semiconductor industry.
Hence, there remains a need for improved methods and systems for generating ultra-pure ammonia for use in the semiconductor and other industries. Preferably, such methods and systems would be configured to meet the increasing demand for high flow rates of ammonia while also providing a safer operating facility that requires less abatement controls.